An MSW incinerator is typically designed to combust a set amount of MSW per day with a given flow of combustion air resulting in a given flow rate of flue gas of controlled composition. It is well known to increase the MSW capacity of an incinerator by oxygen enrichment of the combustion air or by simply reducing the design flow of the combustion air to the incinerator. Oxygen enrichment has the effect of removing nitrogen from the flue gas composition and allows additional MSW to be combusted while maintaining the design flue gas flow rate and concentration of oxygen in the flue gas. Reducing the design flow of combustion air has a similar affect on MSW capacity enhancement as oxygen enrichment, except that it causes a reduction of the excess oxygen in the flue gas. One of the major problems in both of these approaches for enhancing incinerator capacity is that the combustion temperature rapidly increases beyond the physical and mechanical integrity of the furnace as the nitrogen is removed from the flue gas or there is a reduction of the excess oxygen in the flue gas. Further, the reduction of the excess oxygen concentration can impair the combustion efficiency leading to high carbon monoxide levels and can diminish the final destruction of toxic organic compounds. Therefore, there is a great need for an MSW incinerator process and an apparatus for controlling the combustion temperature in conjunction with oxygen enrichment or a reduction in the combustion air flow.
A second problem related to the operation of the modern MSW-to-energy incinerators, is controlling the combustion process to maintain a minimum MSW disposal rate and a constant steam production to a turbine generator, a district heating system, an adsorption evaporative heating or cooling system or other end use. Because MSW is a very heterogeneous fuel which varies considerably in composition, moisture content and heating value, frequent changes must be made to the combustion air rate and the MSW feed rate to maintain the combustion furnace conditions within the designed range. For example, the combustion temperature in the furnace will quickly decrease if the feed changes to a lower heating value, higher moisture content MSW. In such a case, the initial response of the operator is to decrease the flow of the combustion air to maintain a constant temperature in the furnace. However, one is severely restricted in how far the combustion air can be reduced because of:
(a) Permit restrictions on the minimum allowable excess oxygen level in the flue gas. PA1 (b) Permit restrictions on the carbon monoxide concentration in the flue gas which will increase with the reduced excess oxygen.
Even if the operator is able to control the furnace temperature within the design range, the heat release to the steam boiler will decrease due to the reduced heating value of the MSW fed to the incinerator. Unless the operator can increase the MSW throughput, the steam production will be decreased. However, the lower heating value, higher moisture content MSW requires longer residence time in the furnace to achieve the desired total burnout. Increasing the throughput of a higher moisture content MSW while reducing the excess oxygen concentrations in the flue gas to a composition within the permit restriction requirements in order to maintain the desired combustion temperature, often results in an incomplete burnout in the incinerator bottom ash. The net result in such a case is that the steam production to the turbine generator or other end use is usually reduced in response to periods when the MSW is very wet in order to maintain the desired ash burnout and excess oxygen levels in the flue gas. Therefore, there is a further need for an MSW incinerator process and apparatus for controlling the combustion conditions and maintaining MSW throughput and steam production during those periods when the MSW is very wet.
A third problem in the general field of waste disposal is finding economical and environmentally safe methods for disposing of sewage sludge from wastewater treatment plants. While there are many disposal methods in the prior art, the cost and environmental acceptability of such methods are becoming of more critical concern to our society today, especially in urban areas. While sewage sludge combustion is a commercially proven process for such disposal, the high moisture content of typical sludges, which ranges from 80 to 98 wt. % free moisture, necessitates firing supplemental fuels, such as natural gas or fuel oil, to maintain the required combustion temperature. The use of such declining natural resources has obvious economic disadvantages. Co-incineration of sewage sludge with MSW has been commercially practiced. However, as the sludge moisture contents increase, the use of supplemental fuels is required. Therefore, there is a still further need for an MSW incinerator process and apparatus for co-incineration of sewage sludge and MSW which overcomes these obstacles without having to resort to the use of supplemental fuels in order to maintain the combustion temperature at a constant level.
A fourth problem in this field is the disposal from a typical MSW incinerator of the wastewater generated therein in rather large amounts or wastewater generated from another source. Therefore, there is still further a need for an MSW incinerator process and apparatus which has the design feature of eliminating the net wastewater flow from the incinerator or disposing of additional wastewater from other process plants.
H. S. Strauss, J. A. Lukens, F. K. Young and F. B. Bingham, "Oxygen Enrichment of Combustion Air in a 360 TPD Mass Burn Refuse-Fired Haterwall Furnace", Proceedings of 1988 National Waste Processing Conference, 13th Bi-Annual Conference, pages 315-320, 1-4 May 1988, proposed the use of oxygen enrichment to debottleneck one of the oldest operating waste-to-steam facilities in the Hestern Hemisphere. However, the experiments carried out by Air Products and Chemicals. Inc., were limited to low levels of oxygen enrichment because of the design temperature limits in the furnace. The authors discussed increasing the available oxygen in the combustion air to the furnace from 21% to 23%, i.e. a 9.5% increase in O.sub.2 or a 2% increase in O.sub.2 enrichment, which meant that a 9.5% decrease in combustion air should result if all the increase were used and if there were no increase in the oxygen concentration in the flue gas. The authors concluded that this increase in O.sub.2 enrichment would allow "room" for increased throughput of waste. However, increases in O.sub.2 enrichment result in an adiabatic combustion temperature rise in the furnace. For example, if only a 3% O.sub.2 enrichment were maintained, a 400.degree. F. rise in the furnace temperature would result. Such a rise in temperature would exceed the metallurgical limitations in the grate system and the downstream boiler equipment. In fact, the authors state that one of the concerns expressed by the German manufacturer of the grate system in the waste-to-energy furnace was damage to the grates for that very reason. As a consequence, a minimum depth of 2 feet of waste was maintained at all times on the grates during the experiments. Prior to the present invention, oxygen enhancement of 1 to 2% of combustion air to an MSW incinerator was all that was believed to be practical.
The authors of the foregoing reference suggested that dramatic results would probably be gained if the refuse to the incinerator had a very high moisture content such as after a soaking rain or heavy snow. The expectation was that a thinner bed of waste on the grates would result in a better burnout; the ability to direct the oxygen-enriched air to the lower end of the grate would also result in a better burn out; and the fact that less air volume would be needed for the same combustion to "make room" for the extra water in the flue gas which would allow for sustained mass flow of MSW. In actual practice, the authors admitted that any comparison between high moisture contents runs with and without oxygen enrichment was difficult. They did conclude that high moisture content sludge could be run through the system with oxygen enrichment without a degradation of either steam production or ash quality. Although, there is the foregoing disclosure of a combination of oxygen enrichment and use of either high moisture content MSW or large inputs of sewage sludge, there is no suggestion in this reference of the process or apparatus of the presently claimed invention.
Supplemental fuel is required to maintain the combustion temperature within design limits in those cases that are practiced commercially using co-incineration of sewage sludge with MSW. Co-incineration requires the incinerator to be designed for such practice initially, so that additional flue gas flow resulting from the combined sludge, supplemental fuel and associated combustion air are taken into consideration.
In general, MSW incinerators have been designed to enable the incineration of wet MSW with the primary emphasis on the operator's ability to mix MSW with high energy waste such as tires; see page 317 of the foregoing Strauss et el reference. This practice has limited effectiveness, however, since the waste moisture content, the major compositional variable, is primarily a function of the weather in the region which leads to significant variation in the overall moisture content of the MSW. Again, the common design feature to alleviate the problem is to fire supplemental fuel to increase the average heating value of the fuel plus waste being combusted with its obvious disadvantage of economies.
U.S. Pat. No. 3,403,643 discloses a waste incineration process using oxygen enrichment where the oxygen content is claimed to range from 25 to 50%. The air enrichment is disclosed to accelerate the refuse burning and allows one to achieve more complete combustion thus reducing malodorous gases. FIG. 5 of this patent charts the extremely high temperatures attained in combustion and shows increases from just under 2,000.degree. F. to almost 5,000.degree. F. The latter case is based on 40% oxygen enrichment and less than 10% surface water being present in the waste. As set forth above, such temperature increases are well beyond the structural limits of MSW incinerators.
U.S. Pat. No. 3,918,374 describes a multi-stage process and apparatus for thermally decomposing or gasifying garbage in which the garbage is charged to a first stage incinerator gasified by external heat without oxygen. The gas produced in the first stage along with additional garbage is charged to a second stage where the gas is combusted with pure oxygen in stoichiometric quantities. The gas produced in the second stage can, in turn, be burned in a third stage to thermally decompose garbage from the second stage.
U.S. Pat. No. 4,279,208 discloses a process and apparatus in which industrial waste is initially pyrolyzed and a portion combusted in the presence of enriched air. By regulating both the enriched air composition and flow rate, it is stated that it is possible to control the furnace temperature and excess oxygen at optimum levels for the waste pyrolysis. A second enriched air stream is then injected into the gas produced in the first stage during a secondary combustion step in which the temperature ranges from 1300.degree. to 1600.degree. C. It was stated that make-up fuel is required when the heating value of the waste is less than 2500 Kcal/kg.
U.S. Pat. No. 4,520,741 describes a system for incinerating liquid or slurry hazardous or toxic wastes, e.g. PCB's, in a two stage combustion process using oxygen-enriched air. The first stage runs at temperatures in excess of 4000.degree. F. for a very short time, i.e. a few milliseconds, to decompose the hazardous/toxic hydrocarbons into less complex chemicals. The second stage is conducted in excess air at lower temperatures, i.e. 2,000.degree. to 2600.degree. F. for about two seconds to insure complete combustion.
U.S. Pat. No. 4,630,555 describes a discontinuous waste incineration process in which a furnace temperature is lowered to 650.degree. C. (1202.degree. F.) and the waste charge is introduced, pure oxygen is injected at sonic velocity above the waste to oxidize the gas formed from the pyrolysis of the waste and liquid water is used to quench the pyrolyzed waste to limit temperatures to about 850.degree. C. (1562.degree. F.).
The preceding four patents employed a pyrolysis or gasification step prior to a final combustion step. These patents do not suggest a solution to the problems set forth above in connection with oxygen enrichment in a mass burn design because of the differences in waste feed requirements, process and equipment design and operating conditions.
U.S. Pat. No. 4,762,074 describes a process for destroying dioxin and furan precursors and reducing NO.sub.x formation in waste incineration by using oxygen enriched air. Enrichment is specified from 27 to 32% with a minimum combustion temperature of 1200.degree. C. It is stated that there is a reduction of NO.sub.x formation due to the reduction in atmospheric nitrogen during the oxygen enrichment. There is no discussion of the effect of the increased temperatures in the combustion chamber due to the oxygen enrichment on NO.sub.x formation.